As described for example in US2007/0153258, electronic fingerprint acquisition systems have been introduced to eliminate the need to use ink or a chemical reagent. In such systems, the subject's fingertip is placed in contact with a transparent platen. Light is directed through the platen and is reflected from the fingertip to an electronic image sensor. The image of the fingerprint is acquired and may be digitized, analyzed, printed, stored, transmitted, or compared with a previously captured fingerprint.
Fingerprint identification involves the comparison of the many minute and unique characteristics of each person's fingerprint, including the pattern of ridge endings and ridge bifurcations and locations and directions of these features, known in the art as minutiae. Comparisons between an unknown fingerprint and fingerprints of known persons may be accomplished manually or by an electronic system. The process of matching an unknown fingerprint to known fingerprints requires the correlation of a minimum number of minutiae for accurate comparison. In the absence of sufficient clarity and resolution of an acquired fingerprint image, a comparable match may be difficult to make.
In typical devices, a fingerprint image is produced when some incident light from a light source passes through the platen and is reflected at portions of the second layer beneath valleys of the fingertip due to a difference in refractive index between the second layer and air above the second layer. The reflected light associated with the valleys of the fingertip is received by an electronic image sensor. Incident light is generally absorbed at portions of the second layer in contact with ridges of the fingertip.
A large variety of electronic fingerprint imaging devices have been proposed in the art. Many fingerprint imaging devices have a relatively costly platen that is integrated into the device. The platen typically includes an optically transparent rigid first layer covered by a second layer that deforms under pressure from the ridges of the fingertip pressed against it. The deformations under the ridges cause the incident light that is reflected beneath the fingertip ridges to disperse, which desirably reduces the amount of reflected light associated with the ridges from entering the electronic image sensor, thereby making it easier for the imaging device to distinguish between ridges and valleys.
A large variety of electronic fingerprint imaging devices have a common problem of platen maintenance. It is a completely natural and unavoidable occurrence that oil and dirt accumulate on the platen during its use. Environmental conditions such as blowing dust and ultraviolet light can also contribute to a build-up of dirt, grease, or damage on the surface of the platen that obscures fingerprint images. Attempts to clean a dirty platen with a tissue may permanently abrade the surface of the platen and further obscure fingerprint images.
When there are scratches, pits, or other defects on an exterior surface of the platen the incident light that is reflected beneath the fingertip valleys is dispersed reducing the amount of reflected light associated with the valleys entering the electronic image sensor, making it difficult or impossible for the imaging device to accurately identify minutiae on the fingerprint.
In addition to the inconvenience and cost of replacement, scratches, pits, or other defects on the surface of the platen can have a detrimental impact on workflow. For example, healthcare professionals may utilize electronic fingerprint imaging devices for access to controlled medications.
US2007/0153258 (Abstract) describes an optical fingerprint imaging system comprising an optically transparent platen and a protective film removably attached to the platen with an adhesive. The protective film protects the platen from scratches, pits, and other surface damage and is substantially chemically inert to petroleum-based substances which may come into contact when receiving a finger pressed to take a fingerprint image. The adhesive has a selected cohesive strength that allows the protective film and adhesive to be removed from the platen without leaving behind an adhesive residue. The protective film can be replaced periodically as part of maintenance of the platen to allow continued acquisition of accurate fingerprint images. A method for protecting a platen is also disclosed.